Sputtered Piezoelectric Material

ABSTRACT

Piezoelectric actuators having a composition of Pb 1.00+x (Zr 0.52 Ti 0.48 ) 1.00−y O 3 Nb y , where x&gt;−0.02 and y&gt;0 are described. The piezoelectric material can have a Perovskite, which can enable good bending action when a bias is applied across the actuator.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional and claims the benefit of U.S. patentapplication Ser. No. 12/393,644, filed Feb. 26, 2009, the entire contentof which is incorporated herein by reference.

BACKGROUND

This invention relates to forming piezoelectric material.

Piezoelectric materials can generate a voltage differential whensubjected to mechanical stress. Alternatively, applying a voltage acrossa piezoelectric material can cause converse piezoelectricity, that is,the piezoelectric material mechanically deforms when a voltage isapplied. Converse piezoelectricity can cause bending forces in thepiezoelectric material that are extremely high. Both of theseproperties, generating electricity and converse piezoelectricity, areharnessed for use in electrical and mechanical devices, such astransducers, e.g., actuators and sensors. Multiple transducers,including a combination of actuators and sensors, can be combinedtogether in a microelectromechanical system (MEMS).

Piezoelectric materials, such as lead zirconium titanate, can also beused to form ferroelectric RAM (FRAM). The piezoelectric material foreither actuators or FRAMs can be obtained from a sol gel, ceramic greensheets, metal-organic chemical vapor deposition (MOCVD) formed layers orpre-fired blocks of piezoelectric material. However, each method canform piezoelectric materials of different quality and composition. Forexample, a sol gel formation technique may require many individual thinlayers to form a thick piezoelectric material. Also sol gel formationcan leave bonding agents in the final material. MOCVD typically formsthin layers of piezoelectric material and can have very low depositionrates.

SUMMARY

In one embodiment, a piezoelectric material includes a body ofPb_(1.00+x)(Zr_(0.52)Ti_(0.48))_(1.00−y)O₃Nb_(y), where x>−0.02 and y>0.

In yet another embodiment, a ceramic target comprising a ceramic bodyhas a composition of Pb_(1.00+x)(Zr_(0.52)Ti_(0.48))_(1.00−y)O₃Nb_(y),where −0.1≦x≦0.30 and 0<y≦0.2.

A method of forming a piezoelectric material is described. A ceramictarget is biased. The target is in a chamber. A support in the chamberis heated to above 450° C. A deposition surface is supported on thesupport. An inert and a reactive gas are introduced into the chamber tocause ceramic material from the ceramic target to be deposited onto thedeposition surface to form the piezoelectric material.

Implementations of the devices described herein may include one or moreof the following features. For the piezoelectric material, it ispossible for −0.01≦x≦0.15 and 0<y≦0.15, such as for 0≦x≦0.05 and0.08<y≦0.13. The material can have a Perovskite crystalline structure. Ycan be about 0.1. A piezoelectric stack and include the piezoelectricmaterial with a first electrode on a first side of the material and asecond electrode on a second side. The first electrode can include aconductive oxide directly adjacent to the piezoelectric material. Thesecond electrode can include a seed layer adjacent to the piezoelectricmaterial. The seed layer can include iridium. The seed layer can have afilm surface with a (111) crystal orientation. The seed layer caninclude iridium oxide. The first electrode can include platinum. A MEMScan include a body having a compressible chamber formed therein and anactuator adjacent to the chamber, wherein the actuator includes thepiezoelectric stack. For the ceramic target, y can be greater than orequal to 0.08. A seed layer can be applied to the deposition surfaceprior to depositing the ceramic material on the deposition surface. Theseed layer can include a (111) crystal orientation. An adhesive layercan be applied on the deposition surface prior to applying the seedlayer. After forming the piezoelectric material, an electrode can beapplied on the piezoelectric material.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view of a MEMS body with a transducer.

FIG. 2 is a cross sectional view of a MEMS body.

FIG. 3 is a cross sectional view of a MEMS body with an electrode and apiezoelectric layer.

FIG. 4 is a cross sectional view of a MEMS body with a transducer stack.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Forming piezoelectric layers or structures of a device can be done in anumber of different ways, as noted above. However, one method ofpiezoelectric material formation, PZT type formation in particular, isto sputter the material into its desired location. The sputteringprocess and the target used to form the piezoelectric material can inpart determine the resulting characteristics of the piezoelectric layer,for example, the D31 coefficient (the magnitude of the transversecontracting or expanding of the layer perpendicular to the axis ofpolarization in response to a bias voltage applied across the layer) orthe D33 coefficient (the magnitude of the thickness or longitudinalchange along the polarization axis in response to a bias voltage appliedacross the layer). The characteristics of the material obtained bysputtering using a target that includes a specific amount of lead,zirconium, oxygen, titanium and dopant material, such as niobium, can bebetter than those achieved by other piezoelectric material formationmethods. In addition, the plane of the surface on which the material issputtered onto can also effect the quality of the piezoelectricmaterial.

Referring to FIG. 1, a MEMS device having a piezoelectric transducer isshown. A body 100 has a main portion 15 with a chamber 20 therein. Thechamber 20 is open to the environment through an aperture 35 in a bottomlayer 10. Defining a part of the chamber 20 is a membrane 25 that coversthe chamber 20 and the main portion 15 of the body 100. Optionally, themembrane 25 includes a layer 30 that is of a different material than theportion of the layer 30 that is directly adjacent to the chamber 20. Insome embodiments, the body 100 is formed mostly of silicon, silicondioxide or a combination thereof, for example, one or more layers ofeach material. For example, the main portion 15 can be formed of siliconwhile the bottom layer 10 can be formed of silicon or silicon oxide. Themembrane 25 can be formed mostly of silicon while the layer 30 can beformed of silicon or silicon dioxide. The body 100 can be formed byadhering multiple layers together, such as with an epoxy or by fusionbonding the layers together.

A piezoelectric transducer 110 is formed on the body 100 over thechamber 20. The piezoelectric transducer 110 includes a lower electrodestack, an upper electrode stack, and a piezoelectric layer 50 locatedbetween the lower electrode stack and the upper electrode stack.

The lower electrode stack is disposed on the body 100. In someembodiments, the lower electrode stack includes two portions, anadhesion layer 40 adjacent the body 100 and a seed layer 45 on theadhesion layer 40. The two layers of the lower electrode stack are bothelectrically conductive layers. In some embodiments, the adhesion layer40 is between about 50 and 1000 Angstroms (Å) thick, such as betweenabout 100 and 500 Angstroms thick, although other thicknesses can beused. In some embodiments, the adhesion layer is formed of titanium,titanium tungsten, chromium, nickel, molybdenum or another suitabletransition metal or conductive material. As its name implies, theadhesion layer 40 helps adhere the seed layer 45 to the membrane 25.Without the adhesion layer 40, with some types of seed layer materials,the seed layer 45 has a tendency to peel away from the membrane 25 ordelaminate.

The seed layer 45 can be formed between 100 Å and 1 micron thick, suchas between about 500 Å and 5000 Å thick. In some embodiments, the seedlayer 45 is formed of platinum, iridium, or another suitableelectrically conductive material. If the seed layer 45 is formed ofiridium, the seed layer 45 can have a film surface with a (111) crystalorientation.

The upper electrode stack can include an adhesion layer 80 adjacent thepiezoelectric layer 50 and a metal layer 85 on the adhesion layer 80.The adhesion layer 80 can be between about 100 Å and 1 micron thick,such as between about 100 Å and 1000 Å thick. In some embodiments, theadhesion layer 80 is formed of an electrically conductive material, suchas titanium, titanium tungsten, chromium, nickel, nickel chromium orother suitable electrically conductive metal. In some embodiments, theadhesion layer 80 is formed of a metal oxide, such as indium tin oxide,zinc oxide, or other electrically conductive oxide. The metal layer 85that is on the other side of the adhesion layer 80 from thepiezoelectric layer 50 can be between 100 Å and 4 microns thick, such asbetween about 1000 Å and 2 microns thick. The metal layer can be formedof a conductive material, such as platinum, iridium, gold, copper,aluminum or other suitable transition metal.

The piezoelectric layer 50 is formed of a sputtered piezoelectric layerthat is primarily lead zirconium niobium titanate. In some embodiments,the piezoelectric material is a Perovskite piezoelectric material. Insome embodiments, the piezoelectric material has a greater percentage of(100) crystal orientation surface than (111) crystal orientationsurface. In some embodiments, the piezoelectric material has acomposition ofPb_(1.00+x)(Zr_(0.50+/−0.02)Ti_(0.50+/−0.02))_(1.00−y)O₃Nb_(y), where−0.01≦x≦0.15 and 0<y≦0.15. In some embodiments 0≦x≦0.05 and 0<y≦0.10. Insome embodiments, 0.10≦y≦0.15. In some embodiments, y is about 0.12. Thepiezoelectric layer 50 can have a thickness of at least 0.5 micron, suchas about 1 micron or about 2 microns, or greater than about 4 microns,for example between about 4 and 6 microns or between about 6 and 8microns thick.

The piezoelectric layer 50 of the stack can be activated by applying avoltage across the upper electrode stack and the lower electrode stack.Activation causes the combined membrane and piezoelectric layer to bend.An AC voltage can create a pumping action of the cavity 20. If there isfluid within the cavity 20, such as a liquid of sufficiently lowviscosity, the pumping action forces the liquid out of the aperture 35in the body 100.

Referring to FIGS. 2-4, a method of forming the transducer on the MEMSdevice is described. Referring to FIG. 2, the body 100 on which thetransducer is to be formed is provided. An exemplary MEMS body isdescribed in U.S. Publication No. 2005-0099467. However, other types ofMEMS bodies can be provided on which a transducer is formed. Referringto FIG. 3, the lower electrode stack is formed by applying the adhesionlayer 40 on to the body, such as by physical vapor deposition (PVD). Theseed layer 45 is then applied, such as by PVD. If iridium is used toform the seed layer 45, the iridium can be formed with a film surfacewith a (111) crystal orientation.

The piezoelectric layer 50 is then applied. In some embodiments, arotating RF magnetron PVD apparatus is used to form the piezoelectriclayer 50. The PVD apparatus can have a tuned substrate RF impedancematching network for control of the substrate DC selfbias voltage. Asuitable PVD apparatus is described in Physical Vapor Deposition withImpedance Matching Network, U.S. application Ser. No. 12/389,253, filedFeb. 19, 2009, which is incorporated herein by reference. The PVDapparatus can use a reactive PVD process with argon and oxygen gas forthe sputtering gases. A ceramic PZT target that has a composition ofPb_(1.00+x)(Zr_(0.52)Ti_(0.48))_(1.00−y)O₃Nb_(y), where 0≦x≦0.30 and0≦y≦0.2, such as 0≦x≦0.05 and 0≦y≦0.10, can be used with the PVDapparatus. In some embodiments, the niobium content of the target isy=0.1, 0.11, 0.12 or 0.13. The amount of lead is kept below, forexample, 1.30, to prevent forming excess grain boundaries in theresulting PZT. In some embodiments, the target consists of lead,titanium, zirconium and oxygen atoms and no other atomic species.

An exemplary process for forming PZT can have the following conditions.The wafer chuck temperature is between about 550° C. and 750° C., suchas between about 650° C. and 720° C. Higher amounts of lead in thetarget can be compensated for with higher deposition temperatures. TheAr/O₂ pressure is between about 1 millitorr and 15 millitorr, such asbetween about 2 millitorr and 10 millitorr, for example, 2 millitorr and6 millitorr. The gas ratio of O₂/(Ar+O₂) is between about 0.5% and 4%,such as about 2.0%. The cathode RF power is between about 1000 W to 5000W, such as between about 2000 W and 4000 W, for example about 3000 W.The substrate DC selfbias is between about +5V and +150V, such asbetween about +20V and +100V, for example, between about +40V and +80V.An hour of sputtering deposition at these conditions can create apiezoelectric layer that is a few microns, such as up to about 4microns, thick.

To form the piezoelectric layer, a wafer that includes the desireddeposition surface, such as a metallized body, for example, a layer ofiridium with a film surface with a (111) crystal orientation isintroduced into the PVD apparatus. An iridium film with a (111) crystalorientation can be deposited, such as with an argon sputteringdeposition process. If the wafer has a layer of iridium as part of thelower electrode stack, the wafer can be brought to the depositiontemperature, that is, to a temperature of greater than 450° C. and thedeposition pressure. A gas having a small amount of oxygen, such as a 1%oxygen containing gas, can be flowed over the wafer. The low oxygenpercentage oxygen gas is optionally flowed for at least 30 seconds, suchas for a few minutes. The high temperature and oxygen gas causes theiridium surface to oxidize to form iridium oxide (IrO_(x), 1≦x≦3), whichis conductive. The iridium oxide can improve the breakdown voltage of anactuator device. A conductive metal oxide can slow down oxygen loss frompiezoelectric materials, because oxygen atoms are less likely to diffusefrom the PZT into the metal electrodes. After the iridium layer isoptionally oxidized, the sputtering process is begun to grow thepiezoelectric material.

Referring to FIG. 4, the upper electrode stack, including adhesion layer65 and metal layer 70, is applied onto the piezoelectric layer 50. Theupper electrode stack and the lower electrode stack can be patterned, asdesired. Optionally cuts can be made through the piezoelectric layer 50to segment multiple actuators on the substrate.

The piezoelectric material that is described herein is a piezoelectricmaterial that is has a high D31 coefficient. Because of the sputteringdeposition, the piezoelectric material is very homogenous and canconsist of atoms of lead, zirconium, oxygen, titanium and niobium withno binder or other residual material that other deposition processesleave behind. The piezoelectric layer that is formed, such as by usingthe seed layer described herein along with the process conditions forthe PVD apparatus, can have a high percentage of Perovskite (100)crystal orientation. The niobium is a dopant that promotes a Perovskitestructure. A sputtering target with the niobium quantities describedabove can result in PZT films that have a relative dielectric constantin the range of about 1000 to about 1600.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, although PZT for actuators and FRAMs have been described, thematerial can be used for other structures. Accordingly, otherembodiments are within the scope of the following claims.

1. A method of forming a piezoelectric material, comprising: biasing aceramic target comprising a ceramic body having a composition ofPb_(1.00+x)(Zr_(0.52)Ti_(0.48))_(1.00−y)O₃Nb_(y), where −0.1≦x≦0.30 and0<y≦0.2, wherein the target is in a chamber; heating a support in thechamber to above 450° C.; supporting a deposition surface on thesupport; and introducing an inert and a reactive gas into the chamber tocause ceramic material from the ceramic target to be deposited onto thedeposition surface to form the piezoelectric material.
 2. The method ofclaim 1, further comprising applying a seed layer to the depositionsurface prior to depositing the ceramic material on the depositionsurface.
 3. The method of claim 2, wherein the seed layer has filmsurface with a (111) crystal orientation.
 4. The method of claim 3,wherein the seed layer includes iridium.
 5. The method of claim 3,wherein the seed layer includes iridium oxide.
 6. The method of claim 3,wherein the piezoelectric material deposited formed on the depositionsurface has a greater percentage of (100) crystal orientation surfacethan (111) crystal orientation surface.
 7. The method of claim 2,further comprising applying an adhesive layer on the deposition surfaceprior to applying the seed layer.
 8. The method of claim 1, furthercomprising after forming the piezoelectric material, applying anelectrode on the piezoelectric material.
 9. The method of claim 8,wherein the electrode includes platinum.
 10. The method of claim 1,wherein 0.08≦y.
 11. The method of claim 1, wherein −0.01≦x≦0.15 and0<y≦0.15.
 12. The method of claim 1, wherein 0≦x≦0.05 and 0.08<y≦0.13.